1. Field of the Invention
The invention relates to a thermal insulation to be inserted between structures, surfaces, walls of components, and the like that are to be insulated.
2. Discussion of Background Information
In many technical application fields, the problem often exists that surfaces, components, or walls between which a temperature gradient is present must be insulated. This problem especially exists in heat treatment installation such as, for example in high-temperature furnaces. In high-temperature furnaces that are that are operated with air, fiber-like material is customarily used for thermal insulation. These materials have excellent thermal insulation properties, but have the disadvantage that very fine fiber components are released during use. Such fiber fragments have health-damaging effects and, as a result of fiber deposits on the annealing material, cause damage to the product. Besides thermal insulation in the construction of furnaces, such materials play a role in heat insulation of buildings. In further technical application fields, a highly effective thermal insulation can only be achieved with a high degree of expense in the area of production technology. This is based on the fact that a vacuum is commonly necessary between two surface structures to be insulated from one another.
It is the object of the present invention to provide a thermal insulation that has very good characteristics and, at the same time, can be produced in a technically simple and cost-effective manner.
Accordingly, the instant invention is directed to a thermal insulation that includes a fill of hollow spheres composed of hollow spheres that are loose or connected to one another by sintered contacts. Further, a ratio of an outer diameter of the hollow sphere to its wall thickness is within a range of 5-300 and the hollow spheres are made of suicides, silicide composites, metals, and intermetals and their alloys, ceramics, and glass. Moreover, in the case of a closed-pored structure of the walls of the hollow spheres, an internal pressure in the hollow sphere reaches a value that ranges between 0 and 0.1 of the surrounding air pressure at room temperature.
The solution recommended provides that silicides, silicide composites, metals and intermetals and their alloys, ceramic, and glass are used in the form of hollow spheres. Here, it is essential for the ratio of the diameter of the hollow sphere to its wall strength to lie between 5 and 300. The hollow spheres are inserted between the two surfaces, either by being poured loosely or connected to one another by means of sintered contacts. At the same time, these surfaces or strictures to be insulated from one another provide the limiting surfaces of the thermal insulation. The hollow spheres have the advantage that a smaller density of insulation is achieved, lying between 0.1 and 1.5 g/cm3. In using gas-impermeable hollow spheres, with which the ratio between the interior pressure and the outer air pressure at room temperature lies between 0 to 0.1, the convection, i.e., the transfer of heat by way of gas movement, is drastically reduced. Due to the use of suicides and silicide composites as the primary material for such hollow spheres, the possibility is created in the field of high-temperature furnaces of producing a fiber-free insulation material that can be used in air at its temperature of use up to approximately 1800xc2x0 C. For reasons of cost and in the case of low temperatures of use, metals, alloys, glasses and ceramics are used.
Particularly high temperature gradients arise between the interior chamber and the housing of a heat treatment furnace. The thermal insulation consisting of hollow spheres is particularly well-suited for use in such furnaces. It is particularly advantageous for the lining of the interior chamber of the furnace to consist of the same material as the hollow spheres. Certain types of heat treatment furnaces are provided with radiation shielding plates for the purpose of thermal insulation. By filling the present intermediate chambers between the plates with hollow spheres of the same material, an improvement of the heat insulation is achieved.
A further improvement of the insulation characteristics is achieved in that the surfaces of the radiation shielding plates and the hollow spheres are provided with reflection-altering coatings. Due to the coefficients of expansion of the basic material and of the reflection layer, which often do not correspond to one another, an expansion-adaptive intermediate layer is provided that, at the same time, can act as an oxygen diffusion layer. The intermediate layers are adapted in their characteristics to the material characteristics of the hollow spheres and the outer reflection-altering layer. For example, in the case of a ZrO2 outer layer, it consists of a metal-chromium-aluminum-yttrium compound. This intermediate layer prevents further oxidation of the basic material.
Especially in high-temperature furnaces and in the case of possible interactions of the materials with the atmosphere, it is possible to eliminate damages to the material using certain ceramic housings.
The hollow spheres made of suicides and silicide composites as well as of ceramic can be produced by means of powder technology methods. Metallic hollow spheres can be produced by powder metallurgy means or using galvanic processes.
Radiation shielding plates can be produced using powder technology methods (silicides, silicide composites) as well as using casting or deformation metallurgical methods (metals). A particular possibility for producing radiation shielding plates as thin-walled plates is film casting with subsequent separation and sintering. Metal powder spraying and extruding may also be used.
Typical wall thicknesses for hollow spheres and radiation shielding plates lie between 10 and 5000 xcexcm. Wall thicknesses of 50 to 100 xcexcm have been shown to be particularly advantageous.
The use of suicides and silicide composites is particularly advantageous in the production of hollow spheres that are used in an air atmosphere in high-temperature furnaces. At high temperatures and in air atmospheres, silicide and silicide composites form, which prevent a gradual oxidation in the interior of the spheres. If metallic hollow spheres and metallic radiation shields are used that are operated in an oxidative atmosphere, coatings made of oxidation-resistant suicide layers are provided. They may be constructed in a graduated manner for the purpose of functional adaptation. This is particularly the case when, on the one hand, the adaptation of the coefficient of expansion to the basic material and, on the other hand, the adaptation to the chemical reactivity of the surroundings are to be achieved.
Instead of silicide layers, high-melting glasses can be directly used that consist of high-melting oxides such as Y2O3, ZrO2, HFO2, and the like alone or in a mixture with SiO2.
Layers made of high-melting glasses are also provided in the case of thin-walled silicide hollow spheres since, by themselves, they do not form a sufficiently thick glass layer at high temperatures and in an air atmosphere.
The following processes for the production of functional coatings of the hollow spheres and radiation shielding plates have proven to be advantageous: slip casting, dipping in slips or sinters, thermal spraying processes, as well as wet powder sprays.
The hollow spheres can be connected to form semi-finished products in the form of plate-like components by means of sintering. Furthermore, it is advantageous to connect the piles of spheres directly to radiation shielding plates.
The invention shall be described in greater detail with reference to the following examples.
Mo and Si powder are ground into a finely dispersed composite powder in a high-energy mill, with the elements Mo and Si preferably being distributed in a laminar manner and the distance between lamellas being some 10 nm (DE 44 18 598). In this powder, which consists of agglomerates having a diameter of some xcexcm, SiC powder (particle size approx. 1 to 10 xcexcm) is added and mixed into the agglomerates until a homogeneous distribution is achieved. Plates with a thickness of approx. 1 mm are produced from the Mo, Si, and SiC mixture by means of pressing and sintering. The assembly of the radiation shielding plates occurs in correspondence with the construction according to FIG. 2, in which further necessary construction components (rods, pins, wedges, etc.) are produced from the same material, i.e., from the same initial powder, by means of pressing, sintering, and final production. Styrofoam spheres are coated with a suspension, also made of the same initial powders, by means of wet powder vortex layer processes, which suspension consists of the above-mentioned basic powder, an organic solvent bonding agents. After drying, the spheres that have been thus produced are separated by means of a sufficiently slow heating (2 K/min) to 1000xc2x0 C. in an Ar-hydrogen mixture (6.5 vol.% hydrogen). After this, the heating (10 K/min) to 1600xc2x0 C. occurs in a vacuum. After a holding time of 60 min., hollow spheres with a wall thickness of 200 xcexcm are present. These spheres are poured between the radiation shielding plates described above.
Using fine, sinterable powders (approx. 10 xcexcm) of a Crxe2x80x94Ni alloy with poor heat conductivity, the method of production of raw hollow spheres described in example 1 is used by coating of styrofoam spheres with a metal powder binding agent suspension. After drying, the components are separated in an argon-hydrogen atmosphere in the manner described above and subsequently sintered at 1270xc2x0 C. in high-vacuum until a closed-pore wall structure is present and thus gas-impermeable hollow spheres have been produced. After these hollow spheres are mixed with other hollow spheres of an appropriate diameter that allow a maximal filling of space, this mixture is placed between the inner and outer wall of an insulating jug. In this manner, it is possible to achieve a vacuum insulation as a thermal insulation without the conventional fragile glass bulbs being used.